Red skeletal muscle exhibits the widest range of energy turnover of any tissue. It is therefore ideally suited to the study of controls of glycolsis, aerobic ATP production, and the interaction of the two on the regulation of ATP concentration. Present understanding is largely based on in vitro studies designed to solve pieces of the puzzle - i.e. feedback control of phosphofructokinase, control of the redex shuttle in mitochondria, etc. The success of such work permits a synthesis which explains how major pathways interact under physiologic conditions. The principal obstacle to in vivo metabolic studies has been inability to measure a complete set of metabolites including O2 per se simultaneously on a single preparation under various planned conditions of energy turnover. Metabolic and circulatory heterogeneities add further difficulties. The proposed experiments will be performed on a well-characterized dog gracilis preparation containing red fibers exclusively. All supply vessels are preserved in the vascular isolation. Intracellular PO2 and its regional distribution is measured with a cryomicrospectroscopic method with spatial resolution of 2-3 mitochondrial volumes. All assays of tissue metabolites are performed on the same samples used for O2 spectroscopy. Muscles are frozen in situ so all measurements are, in effect, simultaneous. Succunic dehydrogenase is used to scale for the oxidative capacity of each muscle. Thus some concerns about spatial and temporal heterogeneities and biologic variability can be overcome. Intracellular pH, cytosolic phosphorylation and redox potentials, glycolytic intermediates, O2, and parameters related to mitochondrial redox will be determined and correlated with rates of O2 consumption and glycolysis. Measurements will be made at rest, during rest-work transitions, during transitions between steady-state work rates during simulated excercise in the steady state and, if time permits, in recovery. Results will be used to evaluate current ideas of mitochondrial-cytosolic interaction and pathway control in vivo. The data will be used to develop and evaluate quantitative mathematical models of integrated energy metabolism in red muscle.